The technique for chemically analyzing living tissue through nuclear magnetic resonance (NMR) phenomena is well known and essentially includes locating a tissue sample and a transmitter/receiver probe within a strong magnetic field, and using the probe to excite the tissue with RF energy and measure the frequency and strength of the RF energy absorbed or emitted by the tissue. One type of probe used in such magnetic resonance analysis includes multiple antenna surface-coil probes generally comprised of a larger outer circular coil employed as a transmitter and a smaller inner circular coil employed as a receiver. These probes are useful in proton decoupled carbon-13 NMR, DEPTH pulse sequence spatial focusing, Fourier series spatial windowing and one-dimensional rotating frame Zeugmatography. For simplicity, reference will be made solely to the multiple antenna surface-coil NMR probes in one of these applications while it shall be understood that other applications are similarly suitable for similar types of probe designs.
With multiple antenna surface-coil NMR probes, the primary technical problem in the prior art has been the electromagnetic coupling between the two coils. The magnetic field produced by the RF energy driving the transmitter or larger coil has a tendency to induce an RF current in the smaller receiver coil that generates an opposing electromagnetic field which distorts the distribution of RF energy, the magnetic field pattern, of the larger transmitter coil. In a similar, although much less marked manner, the small receiver coil can be thought of as also inducing current in the large transmitter coil. This effect is especially severe when both coils are tuned to the same operating frequency and are co-axially oriented. While this coupling between coils may be measured and adjusted for, it is undesirable in that it generally dramatically degrades the frequency and impedance tuning of the transmitter/receiver electrical circuit, perturbs the desired characteristic of each antenna, complicates the analysis required to interpret the results, and also is an unwanted variable which reduces the accuracy of the data collected.
In the prior art, a number of methods have been developed and utilized for reducing the coupling between coils in a multiple antenna surface-coil probe. These methods typically employ quarter wavelength co-axial transmission lines and/or crossed diodes. Although advances in such coil-to-coil decoupling techniques have yielded improved isolation and workable co-axial surface-coil probe designs, operational problems remain. Non-idealities of transmission lines and diodes lower the antenna circuit quality factor (Q) of the coils. Furthermore, proper adjustment of quarter wavelength cable length and placement in the circuit are non-trivial due to the cable size and required cable change for change in operating frequency. With crossed diodes, separate tuning and matching of each coil requires (ideally) sufficient RF current to short circuit the diodes.
Still another approach in the prior art has been to use two single separate coils fixed in orientation and placed in close proximity so as to form a strongly coupled (high mutual inductance) transformer pair. By choosing the radio frequency circuit resonance mode that corresponds to counter current flow in the two loops, the coupling between a homogeneous transmitter field and the counter current receiver is nulled. Two substantial drawbacks to this approach has made it of limited utility. First, the fixed, nonadjustable (anti-phase) coupling of the receiver elements to the transmitter field presents a true near induced current null only in the absence of asymmetries in the transmitter field over the receiver elements. In practice, such asymmetries are present in most real transmitter fields and, perhaps more importantly, are induced by electromagnetically susceptible specimens. Indeed, distinction of this idealized anti-phase transmitter/receiver decoupling through imbalancing of the transmitter field by the specimen is the working principle behind metal detector gradiometers. However, for the NMR experiment, the signal to be detected arises solely from the specimen after excitation by the transmitter and, thus, the transmitter/receiver coupling must remain nulled in the presence of the specimen. Thus, one must fall back on additional decoupling methods such as the use of cross diodes discussed above. Second, the use of strongly coupled anti-phase loops results in greatly reduced signal detection sensitivity (approximately four-fold) unless the specimen itself presents the dominant noise source in the form of induced eddy currents. In instances where specimen eddy current losses are not the dominant noise source, signal-to-noise is greatly reduced by the strongly coupled nature of the two loops.
As disclosed and claimed in the parent patent, the inventors have previously developed a co-axial multiple antenna surface-coil NMR probe which utilizes typically a larger circular loop for the transmitter coil and a smaller receiver coil consisting of not one but two series connected circular loop elements wound in the opposite direction, the elements being oriented generally symmetrically about the transmitter loop, and in loosely coupled (low mutual inductance) fashion. The probe designs disclosed therein provided for nulling the induced receiver coil current in the presence of the specimen by adjustment of the relative position between the receiver coil elements and the transmitter coil while maintaining their coaxial alignment. Therefore, with the transmitter coil located approximately midway between the two opposed loops comprising the receiver coil, the transmitter magnetic field can be thought of as inducing currents in the two loops of the receiver coil of the same strength but in opposite directions in the single conductor. This results in a net induced current in the receiver coil of substantially zero such that there is no magnetic field produced corresponding to an induced current to disturb the field distribution pattern of the transmitter coil. Furthermore, the second loop added to the typical single loop receiver coil does not enter into the detection operation of the receiver coil in that the second receiver coil loop is sufficiently far away from the sample that it can be ignored. In other words, the second receiver coil loop is beyond the sensitive volume of the sample region of the first receiver coil loop. Thus, the receiver antenna becomes essentially equivalent to the single loop receiver antenna of the prior art that is typically positioned adjacent to the sample. The inventors have constructed a probe which achieves isolation of greater than 40 db between the transmit and receive antennas, and believe that isolation of upwards of 50 db may be readily attained.
While there are many advantages and features of the probe disclosed in the parent patent over the prior art, some of these include the fact that the inventors' device relies simply on the geometry of the receiver coil to achieve decoupling. With such an arrangement, standard, proven surfacecoil designs may be readily utilized without alteration and without the difficulties experienced in the prior art of utilizing particular frequency dependent transmission line filters or diode elements. By utilizing standard proven surface-coil designs, well defined fields are guaranteed to be produced so as to optimize the results attainable from NMR techniques utilizing the present probe. Well defined fields are generally considered as being fields over the region of interest which are homogeneous, exhibit a linear gradient, or which produce typical and well documented field patterns for well known surface-coil type designs, e.g., circular or rectangular coils. Also, the inventors+ design is frequency independent which provides maximum versatility for a probe.
Other embodiments are also disclosed, including embodiments demonstrating asymmetrical receiver coil arrangements, dual element transmitter coils, and other non-orthogonal arrangements which also decouple the transmitter coil from the receiver coil in the NMR probe.
As a further enhancement to the probe designs disclosed in the parent patent, the inventors have found that it is possible to achieve an adjustable decoupling of the receiver coil from the transmitter coil via a change in the angular orientation of at least one of the two coil elements which comprise the receiver coil. In the preferred embodiment, the receiver coil is comprised of a pair of series connected loops. One of these loops is designated the primary loop and is used to sense the NMR effects in the specimen. Hence, it is arranged for placement close to the specimen as the probe is used. The inventors have found that the specimen may in and of itself have electromagnetic properties such that merely bringing the primary receiver coil element into close proximity with it may cause a perturbation in the transmitter field such that it distorts the field and induces a current in the receiver coil. As disclosed in the parent patent, this may be adjusted for by shifting the relative position between the transmitter coil and the receiver coils as they remain generally coaxially aligned. That same balancing may be achieved by angularly rotating the secondary receiver coil element with respect to both the transmit coil and the primary receiver coil element (which remain fixed with respect to each other) to thereby adjust the coupling between the transmit coil and receiver coil in the presence of the specimen. This angular adjustment could be conveniently implemented about either the Z axis or Y axis, presuming that the transmit coil and primary receiver coil elements are coaxially aligned along the X axis. With this arrangement, the induced current in the secondary receiver coil element depends upon the angle between the secondary receiver coil element and the transmitter coil. The induced current is maximal when that angle is 0.degree., and minimal when that angle is 90.degree., while the induced current in the primary receiver coil element is constant. Of course, complete decoupling between the transmitter coil and receiver coil is achieved when the induced current in the receiver coil elements (primary and secondary) are equal and of opposite phase. In order to accommodate a range of adjustment, the maximal induced current in the secondary receiver coil element should be equal to or (preferably) greater than that of the primary receiver coil element.
Although this preferred embodiment is addressed to the particular arrangement shown which, for convenience, depicts the primary receiver coil element and transmit coil in a coaxial alignment, it should be understood to one of ordinary skill in the art that other arrangements would perform equally well. These would include a receiver coil wherein both receiver coil elements are separately rotatable in order to adjust their coupling with the transmit coil. Furthermore, in still another alternative, the field of the secondary receiver coil element could be altered to null out the induced receiver coil current by positioning an electromagnetically susceptible vane or the like therein. This alternative provides the advantage of fixed primary and secondary receiver coil elements which improves the mechanical stability thereof. Still even more generally, the concept of the present invention includes any device or arrangement for altering the electromagnetic response of a receiver coil element in a controlled manner to null out the total receiver coil current induced by the transmitter coil while the sensing element of the receiver coil is located substantially adjacent the specimen for taking measurements therefrom.
In order to reduce overall receiver coil resistance, and also to improve the quality factor and signal-to-noise ratio in the receiver coil, the inventors have developed a parallel connected receiver coil wherein the two receiver coil elements are connected in parallel instead of in series as in the other probe embodiments disclosed herein. Thus, the total anti-phase receiver coil resistance is reduced significantly being less than the resistance of the primary receiver loop alone, which is equivalent to a single-loop coil. This reduction in resistance improves the signal sensitivity of the anti-phase receiver coil. With this arrangement, it is possible to have a primary coil element with a lower inductance than that of the secondary coil and wherein the coil elements are oppositely wound and physically located such that the current induced by the transmitter in the primary element is equal but opposite in phase to the current induced in the secondary element. Therefore, the dual element parallel connected receiver coil may still exhibit the desired isolation from the transmitter coil by having a zero net induced current. However, it is well understood in the art that by way of theoretical analysis the receiver element signal sensitivity is directly proportional to the fraction of current passing through that element when a hypothetical unit current is applied to the entire receiver coil structure. Therefore, as the primary coil has a lower inductance than that of the secondary coil and hence a lower impedance, a greater proportion of the receiver coil current will pass through the primary coil element. This results in improved sensitivity of the primary coil element. As the electromagnetic coupling of the transmitter field to each of the two coil elements of the receiver coil is imbalanced, one of the techniques disclosed herein for balancing the anti-phase currents (i.e., radio frequency currents whose phases are essentially 180 different) induced in the receiver coil elements may be utilized to ensure that a zero net induced current is experienced with the specimen in place.
In practice, the parallel connected, imbalanced inductance receiver coil may be constructed by winding the primary receiver coil element out of thick copper wire which exhibits low RF impedance and winding a substantially larger (or greater number of turns) secondary receiver coil element from thinner copper wire. The imbalance in transmitter field coupling to the receiver coil elements may then be nulled by physically spacing the receiver coil element closer to the transmitter coil. With this arrangement, the secondary coil element will have much greater inductance and impedance than the primary coil element so that more of the hypothetical unit current will be shunted through the primary coil element, maximizing its sensitivity and signal-to-noise ratio.
Although the probes discussed herein explicitly include a transmitter element that excites the sample prior to signal detection by the receiver element, it is understood that the general principles described herein for adjustable asymmetric anti-phase current decoupling of the receiver from the transmitter in the presence of the specimen are applicable to the case of a transmitter structure not formally part of the probe assembly, e.g. a large "homogeneous field" transmitter that surrounds the anti-phase receiver elements.
While the principal advantages and features of the present invention have been described above, a greater understanding and appreciation for the objects of the present invention may be attained by referring to the drawings and description of the preferred embodiment which follow.